thing is , saying you don't remember, or even you blacked out, are common excuses used by people who are guilty.
they believe it somehow sounds less bad, as though they are not responsible for it as they didn't know they were doing it - but it doesn't as most don't believe them, so it just sounds like someone trying to get out of something.
some things are ok to forget and understandable, such as how many times you hit him, or exact conversations - but if you were sober there is no reason at all why you cannot remember if you hit him first.
you also say you did not him, but before said you headbutted him ... so you did hit him.